The original meaning of this word is the one given to 'meritocracy' today. The well-being may or may not be the best, and in fact, according to Plato's views, the 'best' were not those born in the purple.

The word 'king' implies some sort of divine justification for the rule of an individual; kings and emperors in almost all societies, both in the present and in the past, sustain or have sustained their right to rule based on sacred beliefs. The Japanese emperor, the monarchies in Spain, England, the Pharaos, etc. use ore used different sorts of holy justifications; they could be Gods, holy people, envoys, etc. but they always brandish some sort of allegedly sacred right to their position, which makes it indisputable.

Kingship can be hereditary or non-hereditary, and while Plato did not support hereditary monarchies, he did in fact agreed with the concept of holy justification to be used in front of the populace, of the divine right to govern given to someone. His 'noble lie' is precisely this.